Are the smokes on the same circuit as where the iron is being plugged in? Voltage drop at a bad connection, I'd assume, would cause the smokes to dip below operating voltage... I'd assume the "beep" was to alert that the smokes were "without" power... I had a night service call where a homeowner couldn't get one of the interconnected smokes to shut up! Nothing had happened to instagate its behavior.... I got there, disconnected it... still going!, yanked the battery,... now it's chirping... I left it outside while I did paperwork for the customer & installed a new smoke... 20mins later I go out & it's still chirping every 3-5 secs.... I ended up parking my van on it to shut it up!
The customer swore her house had not been built upon any type of cemetary or Indian burial ground!