I misread.
'Hey Graham, do you want to have a drink later on?'
'I'm a little tired, so I'll have a think about it, but I'll probably want to sleep instead.'
'Are you leaving for your trip to Berlin?'
'Yeah I'll be away for three weeks'
'Great, I'll be thinking of you!' - as in you're in my thoughts. You'll be wishing someone well, hoping they're having a good time, and it also denotes a little bit of luck. The act of being thought about, will bring good fortune.
Holly has wrote a better answer than this, but I thought I would add in some key examples. Adding an extra bit of help might be that 'I'll have a think, and get back to you' is a nice common every day phrase meaning 'I'll think about the idea and give you an answer later on.'