How to record chat command location?

I want to record the position I was standing in when giving a chat command.

What's the best way to do this? A short range scan on agents, checking
for the one that is me?

This depends on whether or not you're using the script in an attachment.

If you are, it's easy. Just record the output from llGetPos().

If this is for a stationary object, then yes, sensors are your best option.

Alternatively, you might consider using an attachment AND a stationary object. The attachment would contain a listener, wait for your chat command, and then report your current position to the stationary object. Without knowing your specific plan, I can't offer really good advice here, but if you DO decide to go ahead with this option, remember to use the changed() event with CHANGED_REGION to determine if you're in the same sim as your stationary object. That way, it'll only run the check once per time you change sims, and not leave an open listener.

If I can, I want to avoid an attachment. I want to be able to wander around and record positions in a house/property. The idea is to make it super simple for a user to store that data "/23add foyer" etc.

Start with a listen event.
if string matches up trigger a sensor event with llSensor to scan for said Agent Key.
use llDetectedPos in the senor event to find postion of said Agent Key.