The only way I found this one in a cave, during the day, was that I saw a random medium sized poop. Basically everything in the caves drops small poops so if you see a medium, look for the mega close by. I must have ran by her like 5 times before I realized it was not just a rock lol. The knock-out and tame was easy with 5x taming on my server. Getting her home during was the tricky part.
Useful tip for everyone: Ark’s day and night cycle is 1 hour in real life (on normal settings).
That means if you shoot the dinosaur right out at 8:30 p.m., it will wake up in real time in one hour at 8:30 p.m. the next day. For each past hour you have to subtract 2 minutes and 30 seconds. Example: you knocked him out at 10:30 p.m. -> he will wake up in 60 minutes - 2,5 - 2,5 = 55 minutes
At level 26, it is the first time that the Megalosaurus needs more than an hour to be tamed.. So be prepared to knock him out twice or three times