According to my dictionary*, question is defined thus:
a sentence worded or expressed so as to elicit information
Also, one of my dictionary* definitions of could is:
a modal verb used in making polite requests
Thus, all "Could you…" sentences are really questions because
They are requests, indicating something is being asked for
A response or reply (verbal or not) is required of the person being asked
- 'Could you please pass me the salt?'
'Sure! Here you go.'
- 'Professor Calculus, please could you give me an extension on this assignment?'
'I'm sorry, Isaac, but you will have to turn it in at the same time as everyone else.'
- 'Could you hold this for a sec?' Eric asked his brother.
(Response: Brother extends hand to hold item for Eric.)
- 'Here's the form.'
'Could you sign here, sir?'
'No problem!' (Man signs in indicated box.)
'All set. Thank you!'
Hence, you should always terminate any sentence beginning with "[Please] could you" with a question mark.
*New Oxford American Dictionary (2nd Edition)