The odor of tobacco would be certain to warn any one who entered by means of the front door.
Mr.Warmore was nervous, for the experience was new to him.

He succeeded by a great effort in keeping himself well in hand, venturing only to whisper a word now and then.
"You don't think he is likely to come in the back way ?" he asked in a guarded undertone.
"There is not the slightest danger of his doing so.

That would look suspicious.

He will use the front door, so, if seen and challenged, he will be ready with the excuse that he has called on legitimate business of his own.

At the same time, he will try to manage it so as not to be observed by any one.