If you have to go to the backroom to try to locate a softlines item that's not on the floor, try to pay attention to what all is hanging back there so the next time you can't find something you might be able to recall if you've seen it.
Check the fitting room rack and reshop at guest services. Every now and then I get lucky and find what I'm looking for (one time it was actually IN the fitting room with someone...I waited about ten minutes and came back, and the shirt was hanging on the rack).
For girls and boys clothing, I'll check both departments if it's something like a basic folded t-shirt or sweatpants - sometimes they get put in the wrong place. Same with infant/toddler stuff.
Always check clearance racks and endcaps. Clearance items aren't supposed to be SFS eligible, but sometimes guests like to just throw shit wherever rather than put it back. I've also occasionally had it happen where an item goes on clearance THAT DAY, and the order was from the previous evening. I'm not sure if that's supposed to happen, but it has.
Really, one of the biggest things is to just know when to give up. If you're spending too much time looking for each thing that's not RFID enabled and says it was last received more than a month ago, you won't be able to finish by goal time. Make an effort, but be realistic. Just pay attention to how many things you INF and keep an eye on the INF percentage when it updates each hour.
If you're worried that you're running short on time but you think you might want to spend more time looking for an item, you can always pause the batch and see if you have time to look after you've picked everything else. If you don't, it's easy to open up that cart and just INF the item.
I'm usually by myself for eight hour shifts (usually 8 - 4:30 or 8:30 -5), and if it's under 150 DPCIs when I start I can usually get done on time and keep INF green. It usually depends on how hard it is to find things, how many new orders drop in, and how many OPUs pop up that I have to stop everything and pick. If you're fairly certain you won't be done on time, let the LOD know and they can get you some help (same thing with packing in the afternoon - let the LOD know at least an hour before goal time if you don't think you'll be done. It's their job to get you help). Some days you just get slammed and it's impossible for one person to keep up. It's my biggest frustration with the job, honestly. Every day is different but leadership doesn't always seem to understand that even if the order numbers start out the same two mornings in a row, it doesn't mean you can't be fine by yourself one day and struggling the next.
Good luck!