Theo's relase into the wild was completely not Greg's fault - that had to be made very clear. The poor boy had retrieved a lover from St Mungo's but also a still very sick patient, and while Theo appeared to be lucid at times, to know what he was doing and that he meant the promises he made, he really wasn't; he wasn't that person all the time.
He was exhausting work, and while he would usually wake up in distress, enough to wake his boyfriend up, the fates had decided not to on this ocassion. He woke up with stealth, silent and aware and, unfortunately, completely gone in the head.
Theo hadn't really seen Greg - he'd just seen the still (and at the time) unsual surroundings of the swet little hut and had been panicked by them. This wasn't his head! This was somebody else's head, Merlin, Theo had wandered into someone else's head - shuddering at the very idea, he'd crept out of the window ('I promise, I won't climb out the window, I only want to stay with you, I promise'... or maybe poor Greg had been too tired to remember to renew the locking charm) and had started running, the only thing in his mind to get back into his head, back into his own head before his secrets leaked out like so much yolk from a broken egg. There were things, when he remembered, that he didn't want anyone to find out about him.
By the time his body started to ache from running (which wasn't anymore than five or ten minutes - his body was so unused to actually being used, nevermind the fact that he was still dramatically underweight and sick) he'd forgotten why he was running in the first place and stumbled to a confuse halt.
He wandered like a little lost soul, further into the Forbidden Forest, more defenceless than the youngest student in the school.
He stopped as he saw a thing fly down, sure that it was a thestral and wandring off to try and find it.