It may require a little bit of playing around to set a test up correctly. However, once you have set up a small test scenario that you can move through and clear without drawing enemy ire, reattempting it with the Welrod will result in rapid discovery. Though the AI will usually not be able to find your position after the first shot, they will be in combat mode. Note also that alerting the enemies still happens even when other enemies are not in front of the weapons muzzle.
Mk22, HD and supressed 1911, as well as the suppressed Sten and M3 Grease Gun and M/45 all seem to be quieter than the Welrod for taking out targets.