Flashing question mark inside folder when Mac is left running after some time

I don't get this issue when booting up, only when leaving the computer be for a period of time. When I return, I see the flashing question mark. Shut it down, restart and I'm up and running again. Disk Utility shows no errors. I assume this is a hardware issue of some kind, but wanted to be sure it wasn't a software issue before bringing it into Apple. I am running 10.14.3 now, but it has been happening for a month or so.