I was diving my IDDC char and got the "illusionary castle" theme. Having triggered a few "seasoned traveler" traps already, I was paranoid about traps and started to zap my rod of trap location pretty frequently. To my surprise, I triggered a trap on one of the white #s (illusion wall) one step after detecting traps. I stepped back and no trap was seen. I checked with S, but nothing. Neither with "l"...
So, was that an illusionary trap? Moving on the square triggered the trap again... and I finally managed to sort of detecting the thing by pressing l, p and l again on the corresponding tile (message saying "A trap on illusion wall").
So it seems that the trap symbol is hidden by the wall symbol. Makes crossing those illusionary levels really tedious...