Cat tattoos?

Hi-
I adopted 2 beautiful brothers from the animal shelter on Sept 29. The
shelter told us that they were 2 yrs old and that they were originally
brought in sometime in Feb 05. Then (not sure when), someone adopted
them, only to return them to the shelter in early Sept. After a
check-up, our vet said that they were probably born in Feburary and
were no more than 9 mths old. I guess you can't always trust them to
keep track of so many animals....
Anyway, tonight we discovered some writing on Julius, it looks like a
Sharpie. There seems to be crudely written and large numbers and
letters on his belly, but he won't sit still so we can make it out. My
question is: is this how tattoos are always done (sloppily on the lower
belly), and do shelters do this often? If so, how come we weren't told
about the tattoo? My greatest fear is that some previous owner will
knock on my door wanting them back. Has anyone else adopted a tattooed
cat?
Thanks