Yeah, that shouldn't happen. I just tried to reproduce this but it worked OK for me.
Go to Toad Options -> WIndows. There you can configure Toad to remember (or not) the size and/or position of each window. I have it set to remember everything. I tried making Toad very wide to span both monitors, then open a window and move it to the 2nd monitor. After that I resized Toad so it only was in one monitor and reopened that window. Since the window would have been off screen, Toad put it in the center. I tested using the tablespace map.
Can you look at these settings and give me some steps to reproduce it? If you aren't sure, please zip up your user files folder and email it to me at john.dorlon@quest.com
Thanks
John