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I don't have my phone at the moment, since I turned it over yesterday at the shop for a screen replacement. The SIM card is removed and it's sitting in my wallet now.
When I opened WhatsApp Web a couple minutes earlier today, I was connected to my account and could send and read new messages. This connection continued for around 15 minutes or so, until it stopped receiving signal from the phone.
My question is, how was WhatsApp Web able to receive signal in the first place?
You say WhatsApp web lost signal with the phone, which suggests it still requires the phone to mirror its connection. Might the repair shop have connected the phone to WiFi? If the phone is configured to automatically connect to open WiFi, it could connect without setup on the store's part. I know WhatsApp does not require your SIM card to work, as you can temporarily migrate WhatsApp when traveling and using a new SIM card.
Edit: ran a test where I removed my phones SIM card and connected with WhatsApp web on another device. I was able to send/receive messages.
Edit2: the phone must be unlocked to allow the WhatsApp connection. WhatsApp does not need to be running actively (there is a background process on startup).
At a minimum, your phone was unlocked and connected to (open?) WiFi. It cannot be proven the store was going through your WhatsApp messages, but it is possible they did.
