I love Decepticon nonsense confusing the heck out of Autobots, lol. I imagine they'd still have an understanding of the tension that can develop between nemeses but they would never think of it in terms of an actual relationship. Meanwhile, the Decepticon social structure is such a mess of toxicity that the aforementioned tension is about the healthiest and most fulfilling relationship a lot of them can have.

Anyway, as far as this scenario, I can easily see Starscream giving it away through snide remarks or the pieces finally adding up based on general Decepticon reactions and also the way Megatron treats Optimus after he decides to pursue Ratchet instead. Like. Optimus intercepts Megatron before he can get to Ratchet and is all, "Your fight is with me," and Megatron just sneers and goes, "You had your chance," much to Optimus's confusion. They end up scuffling and Megatron wrestles him to the ground and says, "It's over, Prime," but just turns to leave him there instead of stopping to finish him off. Ratchet arrives to assist Optimus and finds Megatron standing over him and Megatron jumps away yelling about how it's not what it looks like and Optimus means nothing to him anymore. Optimus isn't sure if he should be insulted or what, Ratchet has no idea what Megatron could possibly think this looks like or what else it could be. The rest of the Decepticons have intense debates about whether Optimus is being romantic or pathetic by continuing to pursue Megatron, whether Ratchet is a better match for Megatron or not, whether Megatron is being romantic or pathetic by chasing after someone who's so determined to ignore his overtures, etc. The Autobots keep coming really close to figuring out what's happening but it's so stupid by Autobot standards that they shake it off every time until it gets beyond ignoring.