Illegals signify spies working in deep cover. No secret interaction exists with their intelligence service, neither do they work from an embassy nor possess diplomatic cover. Operations are being carried out by them in a country alone and unlawfuly. At times, this signifies they will just move in and pretend to be a normal individual in the locality.
Just the other day in 2010, it was discovered by the U.S. that a group of 11 Russian”illegals” had adapted stolen identities and returned to Moscow from the 1990s. Spies of this kind could be called “sleepers” also, in case they are only available waiting to obtain instructions on the next move.