Also, it was rather embarrassing.

I took out my pocket file and began to look over my notes.

Then the document was lowered and I was able to get another look at the stranger's face.
He was really extraordinarily like Weiss.

The shaggy eyebrows, throwing the eye-sockets into shadow, gave him, in conjunction with the spectacles, the same owlish, solemn expression that I had noticed in my Kennington acquaintance; and which, by the way, was singularly out of character with the frivolous behaviour that I had just witnessed.
From time to time as I looked at him, he caught my eye and instantly averted his own, turning rather red.

Apparently he was a shy, nervous man, which might account for his giggling; for I have noticed that shy or nervous people have a habit of smiling inopportunely and even giggling when embarrassed by meeting an over-steady eye.