I think this episode may be the result of confusion of dates. On the lead-up to Operation Varsity there was a "test run", a week before called Operation Token, where gliders where towed along the routes planned for Varsity, but they were then not released near the Rhine but were towed back to (or maybe near) their bases. A map of the Varsity planned routes is here (note the leg between Bethune and Wavre which runs a little north of La Louviere):
http://www.airbornetroopcarrier.com/...tevarsity.html
I have not found how many planes and gliders were involved in Operation Token, (it may have been just a few squadrons or serials) nor an exact description of how or where the gliders landed, (i.e. back at the airfields or designated landing areas near the take-off points or what.) The main point is that they returned back along the outbound routes, so gliders may have been observed flying from NE to SW over Belgium.
References to Operation Token can be seen on these links, (search for Token):
(Note this man was based at Woodbridge):
https://aircrew.org.uk/surrey/aircre...es/goughR3.htm
https://news.yahoo.com/hitler-apos-n...180000769.html
The fourth reference to "Token" on this link (taken from "Green Light" about the 81st TCS which was then based at A-80 (Mourmelon-le-Grand Airfield) in France):
https://archive.org/stream/AFD-10092...7-040_djvu.txt
says:
"We performed only one glider training mission, aimed at familiarizing glider
pilots with the hazards of double-tow gliding. Another purpose of this glider
training mission (Operation TOKEN) was to check carefully on the rate of gas
consumption in a C-47 towing two fully loaded gliders."