Can't really know what caused it until you cut the bubble - check whether there's any moisture etc etc.

Unless you find something wrong, though, yes - patch and paint. Make sure you remove any loose paint around the affected area first.

Latex over oil, not properly prepped?

You went away, did you raise the temp in the house? Maybe humidity caused this.

It could be latex over oil paint. If thats the case, there will be more. It could also be a roof leak, and running down the wall, and came through in that spot. Good idea to do some checking before you do the sanding, patching, and priming and painting.