Here's a quote from TK's online check in FAQ:

"14. What do I need to do at the airport?

Check-in formalities including getting boarding card, delivering the baggage and security procedures will be completed at the check-in counters which are special to online checked-in passengers, if these desk does not exists Business class counters can be used instead. (Except Singapur Station.)

Passenger without baggage and have online printed boarding card may go directly to the gate through to security control or otherwise baggage should be registered at “baggage drop off point” or “online check-in desk”.

So, without baggage and with a self-printed bording pass it seems that 'check in' can be bypassed and I go straight trough to security. Since I'll already be inside the airport, 'security' becomes the transfer desk area security.

The weird thing is how people who checked in online are asked to go to a special 'checked in online' line at the check in counters for 'security procedures'.

But then I find this also on TK's site:
In following cases passengers are not allowed to print an online boarding pass.
•Few stations within Turkish Airlines network are not suitable to print online boarding passes.Please click here to find out where online boarding pass is available.
•Passengers without electronic tickets will be entitled to online check-in but will not be allowed to print online boarding passes.
•From the web site and via Call Center purchased tickets must be identical on name and surname with the credit card holder. If this is not the case, online boarding pass issuance will not be allowed.
In a situation like this if the credit card holder is other than the passenger or passengers on the purchased tickets, the credit card holder must apply to a Turkish Airlines sales office in order to declare and confirm the usage of the card.Only having done these steps online boarding pass issuance will be allowed. (Except New York and Chicago flights.)
•Passengers with documental discounts can not print their boarding passes.

I think that's me, although the wording is awkward and unclear.