Tidal heating probably. The closest moon is hot enough from that to glow and that illuminates the rest (out to a specific range at least). I could find them quite often using the search engine so they weren't uncommon.RS 8474-1353-8-11843531-674 is a planemo with 3 moons, the weird part is that it is being lit by its closest moon. I even bookmarked as "How?"
This is a 0.980 find by the way.
Unfortunately (?) I think this is no longer possible in 0.9.9.0 (or at least they're not showing up in the search engine anymore).