The distant AT Guns wish to skeedaddle....

[cjsiam]

So I would propose some wording like this.

When a unit (do we need to differentiate personnel vs. weapons???) has had a spotted marker, due to it's firing or through recce, and is 
currently visible to an enemy unit---It remains spotted as it limbers(as a weapon unit), and as it loads (either?) the transport--as long as they
are in the same hex.  In the case of loading--the spotted markers will apply to the transport--while in the same hex.

At the point of moving to a new hex, the spotted marker is removed, and normal LOS visibility/spotting rules will apply.

This basically reflects that units that have revealed themselves through firing and become spotted are still spotted until they leave the
hex, regardless of limber and/or loading into vehicles....

I think this is within the mechanic, supported easily, and tracks what limitations should remain on units which have
become spotted and known to enemy observers.

Your mileage may vary....