ah! the testosterone is surging through those little loins then! i'm going through this right now and while nips hasn't cocked in my house i'm on permanent "cock watch" (could i have worded that better, perhaps?) elsewhere. on two occasions i've caught his leg at half mast, the latest being on saturday when he decided that my friend's nice bright yellow work bag full of student assignments needed marking.

he's 100& housetrained and has been for some months (he's 9 months old now) but he doesn't seem to make the connection between "marking" and widdling indoors. the latter being something he'd never do.
i don't think there's an instant solution - although if anyone has one please tell me! - and i'm just making sure i keep an eye open for that tell-tale leg being raised and then immediately say "oi, not in here!!" which stops him in his tracks and seems to remind him of his surroundings. he's at his most dangerous in the first 10 minutes or so of being in another house, by the way!