After fledging, martins may come back to their home gourd or compartment for a week or two, while learning to feed themselves. After that, they sleep in trees, under bridges, in cattail islands, corn fields - these martin roosts can remain active from early June to August or Sept, as some birds leave, new ones arrive from points further north.
They are not likely to use different housing for roosting after they fledge - housing is only for the breeding season, and they are too young and innocent to be thinking about
that til next spring

But you can play dawnsong or chatter recordings in mid-late summer to try to get some familiar with your site, just the same.