If a topic only has one user and a mod converts it to a PM, the topic’s OP will be the only participant in the PM if the mod isn’t added to it. The PM will end up in the OP’s sent messages list, but the OP won’t receive a notification telling them to look for it there.
The situation where I’ve had to convert public topics to PMs is when something gets posted publicly on the forum that should be dealt with internally. If mods are handling this kind of situation it makes sense that they get added to the PM. If mods shouldn’t get added to the PMs, maybe the Make Personal Message action shouldn’t be available to them.