For $10.99 at Lowe's, suggest you get a roll of 3M high temperature flue tape, high heat sealing tape up to 600 degrees, 15-foot roll x 1.5", silver color and add some more tape to the metal exhaust pipe behind the boiler, especially where it makes a 90-degree turn as shown in your photo. A very small and smelly amount of exhaust could be getting out, there, where the exhaust vent pipe hook-up takes a sharp turn between the boiler and the brick chimney.
https://www.truevalue.com/15-ft-hi-t...lver-flue-tape .... a fix for leaky chimney pipe connections where one section fits into the next with a friction fit. To test for an exhaust leak, simply hold your hand up to the joint connection.
Google this: "Even Low Levels of Soot Can Be Deadly to Older People, Research Finds" ..... January 26, 2022: NY Times ..... especially the reader's comments down the bottom