ssh wants passphrase and password if accessed via remote VPN login

[MR ZenWiz]

I have an Xubuntu 14.04.4 workstation at work as the base for most of
my activities.  The actual work must be done on either Windows or a
CentOS machine, and I have a dedicated CentOS 6.7 box for this.  I use
ssh to log in on the CentOS box, and all is usually as I expect - no
pass phrase, no password needed for the login.

In working from home over the last few days (and a month ago, I
think), I use a Win7 VM for VPN access to log into the workstation
remotely with PuTTY.

That works fine, but when I ssh to the CentOS box, ssh asks for my
pass phrase and password on every login.

What would cause this for a remote ssh but not a local one, from the
same originating box?

Thanks.
MR