I connect one of these up when ever the bike is in the shed. The open circuit voltage is always 12.9-12.7 volts and it stays like this for days with the charger only monitoring the voltage and not actually charging.

Have a read through this Yuasa PDF Section 3 is all about Inspecting, testing and charging.

Methods of Checking Battery Condition
Hydrometer Voltmeter

100% Charged 1.265sg 12.60v
75% Charged 1.210sg 12.40v
50% Charged 1.160sg 12.10v
25% Charged 1.120sg 11.90v
0% Charged less than 1.100sg less than 11.80v