In-universe, it makes perfect sense for Odo to fake it. After all, he has caught Croden with a dozen lies, but suddenly he seems to be telling the truth and claiming he's a decent guy after all. This has to be put to the test somehow!
That B-4 glows like a beacon is excusable, because none of the heroes ever go "Ah, positronic screams - it must be another you!". Instead, the very existence of the signals is a mystery, quite probably associated with Soong's antics, but not immediately identifiable as a Soongian android.
When the heroes do find the android, the mystery just deepens, so commenting on an anomalous signal there and then isn't to be expected. It's quickly obvious that B-4 wasn't put together quite right, so "leakage" as an explanation might occur to the heroes soon enough, and need no further comment.
Whether Soong actually had anything to do with B-4 is debatable. Romulans were planning on replacing Starfleet personnel with duplicates; perhaps they also chose to build a duplicate Data? B-4 might be the best they could come up with, and Shinzon would gain access to it when searching the vaults of Tal'Shiar either after his ascent to power, or during the process of his Spartacus revolt. Or even during his prior military service at that. Being a clone of J-L Picard might have made him the smartest jarhead in all of Remus, and well positioned to recognize opportunities like that...
But again, not really plot holes, since these things didn't affect the plots one iota. Conscious or not, Odo would have gotten out of that cave with Croden. And leaking or not, B-4 would have been viewed with suspicion yet taken aboard nevertheless.
Timo Saloniemi