[mistyvales]

Odo spies on Quark in Season 2. Think it was the Maquis two-parter? He doesn't care about Federation ethics early on though.

[varenc]

Hah but the majority of the time Odo spys on Quark by morphing into a common place object like a table or glass. My new headcannon will have to be that Quark keeps removing Odo's spy sensors. Odo definitely would use them if he could.

[pndy]

Pretty sure Odo spied on Quark all the time - kinda weird but useful hobby