OP here - thanks everyone - it's really a question of whether I need to rent a car for the day - I can, but it's a bit of a pain, and I am prone to getting lost. I'd rather take a taxi to the agent's offices and have them drive. It sounds like that's an acceptable practice, but I take the point that choosing an agent in Potomac and asking them to pick me up in DC is asking a bit much. I will try to find a DC-based agent licensed in MD.
Just to clarify (especially for the last poster - glad to see you're rather nasty-jump-to-conclusions comments are the exception rather than the rule here), I don't mind using an agent - in fact, since we'll be new to the area and relocating from overseas, I
want to use an agent. In this case, I am taking advantage of a business trip to take a quick look at a house we've fallen in love with on line - it's a one-off, and a longshot (especially since it's priced a bit above our budget). We had planned to rent for a year after arriving, interview and choose a good agent, and then start a search in earnest during that time. I don't want to tie myself to one agent chosen in haste from afar to look at this one house, so my earlier post was about that. I got a load of positive, constructive and useful advice from this board, and I am very appreciative. On that basis, I understand I can pick a reputable agent and ask him/her to show me this one house. If we buy, we would certainly use that agent, who won't have done all that much to earn a commission, but I expect that's the nature of the job - sometimes it's "easy" and sometimes its a PITA, and if you're lucky, it all comes out ok in the end. I have zero issue with that agent earning a nice commission on an easy sale. If we don't buy this one house, then we might use the agent (if I were lucky on the choice and it's a good fit) for a broader search later on or find someone else. I believe based on advice here that this is perfectly acceptable. I will certainly be upfront with any agent I ask to show me this one house to be clear that it's a one-off.
I like the idea of looking at a couple of other houses while we're at it, and there are a couple of others in the same neighborhood that would be good for comparison. I don't have a ton of time (just that Sunday) and also don't want to waste an agent's time. But one or two would be useful. Thanks for the suggestion.
Again, thank you ALL (one exception :roll

for the useful advice, yet again.