What I am doing right now, to rule out a wifi breach like you, is to replace my normal AP, with a raspberry pi using a TL-WN722N and acting as the normal AP.
This means using the same ESSID, same channel, and mac address (BSSID), and passphrase, same frequency, and same type (wifi n). I am using create_ap ( a simple bash script) to do that. You don't have to reconfigure any of your device.
That way. I am able to keep a log of all associated device with the AP. Wether they use DHCP or static IP. Moreover, I disable wifi on the device I don't use to be sure their mac are not spoofed, and try to keep tab of when I use wifi on those device or not (which is quite harder to do and a hassle).
I am not running frame capture on the rpi for now. I could use tcpdump or tshark. But I am not, for now.
Of course, you can't go back in time and get data that has not been collected. So the first answer still stands: it is not possible unless you had some device that logged the activity you are looking for.
Your best bet is your router log (for DHCP like you said) or your AP if it's not the same as the router.