|"Tethering also turns your phone into a mobile Wi-Fi hotspot, allowing you to share your Web connection with those around you."|
Does not sound like your neighbor is doing anything wrong when his equipment tried connecting to your wifi hotspot. If anything your neighbor might have seen this as a compromize of his network which is why its not happening now.
It would not be the same if you saw his laptop connected.
Your antivirus, antimalware checkers should have root kit and trojan detectors.
Your software firewall should be notifing you of any outgoing or incoming issues it sees as supicious.
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