Depends on what visa, as prehaps they have terms and condtions on their visa. My Wife has come and gone to france once, italy twice and been back to phill.
She went with out me on one trip to italy and i was a little worried as all her visas british and scehgen were tied up with me but she was fine.
I guess it depends on the visa, length of time they went abroad i guess.
I know of one guy who had a problem but he was not suposed to be here in the first place as he had applied for ayslum when he shouldn't have due to his nationality etc.
I saw in the paper the other day that since fingerprints were brought in for visa applications, 500 applications have been stopped already for one reason or another.
I was speaking to a guy who owns a language school today in london and we were just chatting when he got a call from one of the the teachers that none of the people on his class had turned up.
I said why is it a bug, he was like oh no they are looking for new jobs, the hours of their job had changed or that they have moved on to somewhere else in the UK. Most were on student visas....