Well, if you were on a training mission in a stealth aircraft flying with drop tanks and did not drop them prior to entering rival radar range, the radar of a rival nation would see your drop tanks and pylons, but not your stealth fighter (if it works) BUT they would be able to assume that those drop tanks and pylons are attached to a fighter, no? Then, they could analyze the return pattern and see if they can pick up any minute traces of signal (normally considered background at that level because it's so minute) around the drop tanks/pylons and see if they could detect a pattern. Then, they could program their radars to call a positive on that pattern whenever it detected it, even if it was way below the normal threshold for a call. I'm assuming, of course, that stealth fighters do not return no signal at all, but rather return a signal (in a unique signatory pattern depending on stealth model) too small for the radar (under normal circumstances) to determine from background noise.
I don't really know if they can do this; this is just what I suppose may be possible so do correct me if I'm wrong.
Nah!, that there is dreaming, LOL