I think it is totally fine to come back after you are alum. (oh BTW Phi Sig doesn't make anyone go alum you can be in grad school and still be "active status") Most alum from my chapter are at the getting married having lots of babies stage in life so they don't come around that much. But we do have reccent alum come back to lots of stuff and I think that is awesome.
I know there has even been debate on my campus about not allowing alum to come to rush. (its hard when you only have two sororities in Pan and they each feel completely different on the issue.)
I say if they want to be there let them, and don't treat them weird. (but like you all mentioned if they are going out and partying when they are 40 and hitting on someone that is 21 I think I'd do this:
)