I agree with Jake. If I wondered off and then started getting impersonal automated emails, it wouldn't enamor me in the slightest.
But, I think you could take their cue from whether or not they left a public email address. Leaving a public email, basically means, "OK to contact"; no public email means, "do not disturb".
If someone I interact with a fair amount seemed absent for no reason, and they left a public email address on file, I would email and check on them (so to speak) or as some have done, just post a public query of "wonder what happened to X?".
My 25 cents worth.