I did not find an online way to get the address of a BoC ATM from an ATM ID but I can tell you what I tried unsuccessfully and successfully to get the address:
Going to a Bank of China branch
At least in Guangzhou, this is painfully slow. A security guard must swipe a card in the machine that emits a slip with a number. There are a small number of tell windows. It took about 40 minutes in the nearest branch to my accommodation.
When your number is called it turns out that even an employee in a bank of BoC cannot find an ATM street address from an ATM ID. They will point out that there is a phone number on the ATM receipt:
It turns out the call centre is available outside banking hours. I don't know if English speakers are available as I got an employee at my accommodation to call. This successfully resulted in getting the ATM address.
On cards retained by Bank of China ATMs in China
I didn't notice the pink details printed on the ATM receipt with my bad eyes until now but point 3. is relevant:
If the card is retained by the ATM because of any reason but the request from issuer bank, please claim your card with presence of the personal valid certification in the acquirer bank in 3 working days.
Basically I lost a day since I didn't get to a branch until the afternoon of the third day, which took so long, that there was no way to get to the branch that had my ATM card before closing.
Today is the fourth day. I got to the bank just after opening. My card was in the ATM. I did get it back. But it was already blocked.