High ambient temperature can cause batteries to self-discharge much quicker. 10 degrees hotter doubles the rate.
In Germany we had a very hot summer and many batteries of seldom used vehicles died because of deep discharge.
Today I checked the batteries of my 4 bikes. The TDM was last time used about 7 weeks ago. The voltage was only 12,05 V.
NTV and Fazer had 12,50 V after about 4 weeks. The BMW has a special battery (originally designed for uninterrupted power supply of alarm systems) and after 3 weeks it had still 12,8 V.