The 18-year-old, understood to be suspected of planting the device on a Tube train, is believed to have been living with foster carers Ronald Jones, 88, and wife Penelope, 71.
Their home was subject to an armed raid on Saturday morning and is still behind a huge metal police cordon on the street in Sunbury-on-Thames, Surrey.
Stephen Griffiths, 28, who lives opposite the "lovely couple", said they were visited "multiple times" by police, and added that he thinks their house may have been under surveillance.
"The police were there multiple times over the span of about a month - a few times a week," he told the Press Association.
"They started off as normally dressed cops, then moved up in the police ranks, wearing black uniforms in an undercover car.
"They used to speak to Penny and Ron on the doorstep, but the last couple of times they went in the house.
"You always think foster kids are going to have a bit of trouble, but you don't think terrorism. It's crazy to think it's over the road from you."
Mr Griffiths said the home was last visited by police between two and three weeks ago, and having witnessed Saturday's raid, he now believes they were counter-terror officers.
"You need to question whether the house was under surveillance," he said.
"I think counter-terror police visited a few weeks ago, and if so, why wasn't something done sooner?"
A Metropolitan Police spokeswoman said the 18-year-old, who was detained on Saturday morning in the departure area of Dover ferry port, had not been arrested "in the last couple of weeks".
But she could not confirm whether he or the property had been visited by officers recently.