Two aphorisms are cited in tandem by both Mark & Luke right after the interpretation of the parable of the sower:

• an observation about the placement of oil lamps &

• a declaration about the disclosure of secrets.

The only logical link between these sayings is that they are both generally concerned with getting something out into the open. Otherwise there is no common catchword or rhetorical pattern that accounts for their conjunction. Nor is there any inherent connection between these sayings and the preceding motifs in this section of the gospels to explain their placement at this particular point in the Markan & Lukan narratives.

Although neither is presented at the parallel point in Matt 13, variants of each saying are found separately in earlier sections of Matthew's narrative. The same observation about oil lamps is made right after the beatitudes at the beginning of the Sermon on the Mount [Matt 5], while a variant declaration about revealing secrets is located about the middle of Jesus' missionary instructions to 12 disciples [Matt 10]. 

So, any theory of the literary relationship between the synoptics must take into consideration Matthew's divergence from the others regarding the linking & location of these two small aphorisms.

To complicate comparison of the synoptic texts further, Luke includes two versions of each saying.  Moreover, his second set is recorded separately -- as in Matthew rather than Mark.  Yet, while Luke locates the first pair of sayings at a point in his narrative exactly parallel to Mark's, neither of the second set of sayings is presented in a literary context parallel to Matthew's. Rather than mimic Matthew by introducing his set of independent aphorisms well before this chapter on parables, Luke records them only much later -- in conjunction with sayings Matthew did not associate them with.  

The lamp aphorism is only one of many sayings in Matthew's Sermon on the Mount that is not found in Luke's Sermon on the Plain [Luke 6], even though the structure and contents of the latter are generally parallel to the former.  Instead, Luke appends his doublet of the lamp aphorism to an ominous warning about a "sign of Jonah" [Luke 11:29-32] -- another non-Markan saying which Matthew reports [Matt 12:38-42] just prior to the pronouncement story about Jesus' true kin.

Likewise, Luke's report of Jesus' missionary instructions to 70 disciples [Luke 10:1-17] is similar to the first half of Matthew's instructions to the 12 [Matt 10:1-16].  But this is not where Luke puts his second version of the saying about the revelation of secrets.  Rather, in Luke this aphorism, along with others that Matthew linked to it [e.g., an aphorism about rooftop proclamations], forms a separate pep talk to Jesus' disciples about opposition & persecution [Luke 12:2-9]. He places this after a series of warnings concerning scribes and Pharisees [Luke 11:37-54]. Matthew also reports these warnings [Matt 23:23-39] but only after Jesus has come into conflict with authorities in Jerusalem at the end of his public career. 

So, Luke's parallels to Matthew's version of these two aphorisms are as complex as his parallel to Mark's text is simple. 

Varied patterns of verbal parallels in the four versions of each of these two sayings further complicate the picture of the relationship of these synoptic pericopes to each other.

The Lamp

Mark's version of the lamp aphorism is in the form of a rhetorical question. Matthew's & Luke's are pronouncements.  

The only words common to all versions [red text] affirm that the proper place for a lamp is on a stand. To emphasize this point all versions preface it by noting the absurdity of putting a lamp under something.  But there is considerable variation between these texts in identifying the places that are inappropriate for a lamp.

• Mark & both of Luke's versions reject a pair of locations; Matthew, only one.
• Matthew, Mark & Luke's second version which is not parallel to Mark agree [blue text] that lamps are not put under a bushel.
• Mark & its Lukan parallel agree [teal text] that lamps are not put under a bed.

Luke's two versions of this saying reject different pairs of locations (vessel/bed; cellar/bushel). But both frame the proclamation about the proper setting for a lamp with the same rhetorical formula [purple text] that is not used by either Matthew or Mark: "No one after lighting a lamp...that those who enter may see...."

Secrets Revealed

The second aphorism is a balanced couplet which uses parallelism to reinforce its claim that the public disclosure of secrets is inevitable. But there is wide variation in phrasing & terminology.

The only word common to all four versions of this saying [red text] is: hidden [κρύπτον]. And even the position of this word varies. In Mark & its Lukan equivalent it is used in the first stich, while in both Matthew and the parallel version in Luke it occurs in the second. 

Given such varied performances, it is not surprising to find verbal differences between Mark's version of this saying & the aphorism at the same point in Luke. It is all the more noteworthy, then, to find that Luke's second version of this aphorism is virtually identical with Matthew's, even though they are not set in the same narrative context. Such verbal agreement is even more striking, if one notes that Luke's version of the saying concerning rooftop proclamations, which he -- like Matthew -- appends to the aphorism about revealed secrets differs from Matthew's text not only in narrative setting but phrasing & logic.

While such rhetorical variation does not measurably alter the meaning of these sayings, it needs to be taken into account by any source theory that claims to explain  the relationship of the synoptic gospels.