I travel for work a lot and often go places early, and will sometimes add in stop-overs that are only semi relevant to my work.
The stop overs are generally because it means I have a shorter flight to my final destination, and thus I can hit the ground running a lot easier. So if possible I might go somewhere 5-6 hours away in the right general direction for a day or two, and just prepare, maybe network with whatever team is around, be there to consult if wanted etc. It makes the next leg of the trip less daunting, which is really helpful if you're essentially going straight into meetings or dinners.
Sometimes it's just what works with flights too. Occasionally direct flights on the right day all arrive at awkward times or are stupidly expensive, and if you've got nothing keeping you at home it's just easier/cheaper to go early or go somewhere else and leave from there.
And yeah, occasionally I can swing it so I end up going somewhere for no serious reason beyond I like it there.
So I don't know OP. It certainly could be dodgy, but if she's a regular traveller it sounds pretty normal to me. Is there any real reason your DH would have been worried you'd find out if he didn't tell you eg. you follow her on social media or you're friendly with another colleague who's going too? If not I can't see why he'd tell you all about it if he was in fact planning on hooking up with her.