Raising temps

I am in a quandary about a situation where our three thermostats are showing an increase in temperature despite being on vacation and set up for 83 cool and 60 heat while the outside temperatures are ranging from 68 to 72. Currently after bringing everything down to 76 weve gone back up to 78° With the above vacation settings. We have asked our cat sitter to check that fireplaces, ovens and ranges are not on and he has found nothing to explain the increase in temperatures in all three thermostats which are on hold. As a longshot I am asking you do you have any ideas.? We will be home in three days so perhaps we will then get to the bottom of this phenomenon.

Solar heat gain. With sunny days my home will rise dozens of degrees over the outside air temperature

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I agree that this is pretty normal.  It also illustrates why setting wildly different temps for your HVAC while you are on vacation might not actually save money.  If you are going to be away for an extended period of time (a week or more), then it probably will save money.  But if you are gone for a weekend and your temps rises from a regular temp of 70 to 80, having to run the HVAC for hours to bring the temp back down to 70 might actually cost more money than leaving the temp at 70 (or maybe going to 72 or 73).