Over the years I have sold a lot of home that were vacant (unoccupied) while I marketed them for my clients. I learned quickly that even vacant homes need attention.
No one is living in the home but they still get dirty from buyers and their agents walking through. Dirt and debris gets tracked inside, bathrooms get used, fingerprints appear on glass, etc.. Insects find their way into vacant homes and a lot of them never get out alive, dead cockroaches in the middle of the room make a poor first impression. Spiders slowly take over corners of rooms and ceilings and spin their webs. Lights inside and outside get turned on and left to light up the house and yard until a neighbor goes over and turns them off or I go by the house to check on things. Doors get left unlocked and sometimes left wide open! One afternoon I discovered an open screen door to the backyard and a snake on a kitchen counter!
The nose knows. When a house is closed up it can start to smell stale. I usually recommend to my clients that they keep the fans running on their HVAC systems. If its nice out I will try and get over myself and open some windows. Air fresheners are sometimes necessary but they can be overpowering.
Outside of course the yard and landscaping keep doing what they do, grow. Depending on the time of year, grass may need to be cut 2-3 times a month. Gutters, roofs, driveways, walkways and yards can quickly become littered with leaves, acorns, pine straw and branches. The outside of the house is the first thing buyers see when they pull up into the driveway and its got to look good.
Bottom line is if your house is for sale and you are not living there, if possible try and get at least a couple of people to watch over it. Your agent should be checking on it at least once a week. I have been know to vacuum my vacant listings, spot clean and blow driveways and other areas outside that start to look unattended. I also call yard maintenance contractors and house cleaners on behalf of my clients when things really start looking bad. Consider also giving a neighbor a key and asking them to stop by weekly.