Even if your fire marshalls use a sweep system in your main building, why can't the lone worker report to somebody during an evacuation? That seems the most logical step and it avoids a search party going out to find him.

We do already have arrangements in place for the worker to report to nearest fire marshal on evacuation, but my issue is with going in to a building to find him if he doesn't show that may potentially be on fire. Everything I seem to read advises against going into a building once the alarm has been raised, but I'm not sure how we stand if he doesn't show. Appreciate the replies so far - thanks.

It seems to me that you have done enough if your procedures say he should report to a warden upon evacuating this separate building. In the event that the fire is in the second building, you can make a choice that the procedure should send two people to look for him - subject to a continued assessment that their environment is safe to proceed, or simply inform the fire service that one person is unaccounted for. It all depends on the size and layout of the building, what use the building is put to,and how long the fire service are likely to be before arriving. It might be reasonable to go looking for this missing chap in a small single storey low risk storage shed, but not so reasonable if its a 4 storey chemical store and your site's 100m for the fire station!