Trolls already use continuous transformation on themselves; to find a needle in a haystack with a magnent, you first have to suspect there is a needle. Harry's partial transfiguration attempt(did that happen or...?) is probably more like a needle in a haystack of troll-magic. The only people who I think could spot it without knowing what they're looking for...already know about partial transfiguration.

Edit: and then Dumbledore finite's the acid to prevent transfig. sickness. any chance of anyone else figuring it out by looking at the magic involved will be looking for a needle without a magnent.