I would guess this is probably down to the volume of people through the lounge - as a generality, of course, it's much greater through T5 than T3. As technically both people should enter the lounge together, I can imagine in an informal arrangement such as you've described it would be harder to remember exactly who asked to guest whom, plus the reception staff do seem to turnover reasonably regularly in T5 on a given day - so just leaving the details noted down for the next employee to pick up might have been problematical.