Presumably you've got evidence that this employee is using his time out of the office to do something extraneous to his job description: complaints from customers about missed appointments; undelivered work; an inability to contact him where you expect him to be.
You need more than just a suspicion based on gut instinct. Start cross-checking your various concerns until several different bits of evidence point to the same conclusion. Even then, don't make assumptions, or you will start to see evidence in the way you want to see it, rather than viewing it objectively.
Having got to the point where you are pretty convinced that there's something amiss, it's time to confront him. Call him into a private meeting place and explain your concerns to him, without in the first instance presenting all the evidence you have amassed. Something simple like: "I've been concerned that we've not always been able to find you where we thought you were. Can you explain that? We've got a duty to be concerned about you and know what you're doing."
This may result in a perfectly innocent explanation. Maybe he organises his extra-office activities differently from the way you expect. Maybe the communication lines aren't clear. But he might also say something along the lines of "I produce the results you want and surely that's good enough".
You then need either to agree better ways of distance management in the future, or, if the situation is more serious, he needs reminding that he's not on a contract only for outputs, but to be available for stipulated hours. Either way, you can and should enforce more rigid reporting-in arrangements until you are satisfied the position is back under control. And the evidence you have can be helpful in telling you how to manage this.
It's more serious if he denies the problem. Then you'll have to confront him with the evidence that you've amassed and ask for his explanation. There may be a perfectly innocent one, in which case you simply have to decide what needs tightening up. But if you are not convinced by his responses and believe the organisation is suffering from his absences - and that the matter is perhaps having an adverse effect on colleagues - then disciplinary action may be the only answer. Just the threat of it may produce the improvements you want.
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