Don't think they are designed to be opened from the outside....
I had a wee shuftie past the one outside Hope Street Post Office, similar design to these two outside ma work. They've all got road-barriers next to the hatches and blue/grey vent cabinets nearby. Should be easy to spot.
Anybody seen any others? End of the tunnel in Cathedral St, probably has a similar looking hatch (hinged, steel, cross hatch design).
Washington St
Opposite Marriott Hotel Argyle St
Haven't got a picture of the Hope Street one as it was a bit busy up there and I haven't quite got over my fear of taking apparantly useless photos of the pavement in public yet ! Funny how passers-by always stop to look at what someones taking a photo of !