I very much doubt that I saw the same animal; although it was the same size as the first sighting. It appeared in a secondary school I was visiting. I think it came from an ornamental pond, although I did not see it leave the water. The lizard entered the school through an open door and promptly climbed a staircase up to a walkway on the first floor. I heard various screams from some of the girls, but I was on duty downstairs so I did not have to go and help! The animal then reappeared to me on the edge of the balcony of the first floor. It waited there for a little while until it decided to jump down in one leap. It performed a loud 'bellyflop' on the tarmac below, and then ran off into some bushes near the building. I didn't see it again. I am surprised it could still walk after a leap like that!
So, after a few months (in August 2016) I have just seen a specimen of about 2 metres in length in a ditch in the Saphan Mai district of Bangkok. It was very shy when it saw me, but I had time to get a few photographs. Water monitor photo