I appreciate it when people randomly reach out. I figure my friends are probably the same way, so I've started getting in the habit of texting people I haven't seen in while when I know they're nearby, even if it's just for a quick hello. Because expectations are low, it's easy to come away from these short and sweet meet-ups feeling good. Even in the worst case when they aren't free, it's no big deal because it was last minute.
I've been pleasantly surprised that people are able to make time to catch up more often than I expect. Since people don't advertise lulls in their schedule, the only way to find out if you have any overlaps is to ask. With ample notice, friends can hold off on taking breaks from work, so you can spend some time together.