About a week ago, I was standing in my kitchen in the morning so the light shines into the house at an angle and some sunlight lit up a drip on the wall next to the kitchen. I went over to it to see if it was water leaking in through from the roof or anything and i followed the drip up the wall with my eyes and about a head higher than mine was a Christian cross shape daubed onto the wall and the drip had run down from this. I touched the drip and it's some kind of oil, very light, with no scent. It seemed quite strange as I hadn't put it there, neither had my partner or anyone who'd been at ours in the last few weeks. Then, day before yesterday, I was in the hallway outside our bathroom brushing my teeth (again, in the morning before going to work) and I turned to walk back into the bathroom when I froze... There, up on the door, was another cross. It then occurred to me that the bathroom is a rather random room for there to be one and it was then I realized before I even had to check, that if it's on our bathroom door, then it's on all the doors. And sure enough with a slightly sinking feeling in the pit of my stomach I checked the other doors: the toilet, our bedroom, our 2 spare rooms, the front and back wooden doors ALL have an oily cross fingerpainted onto them.
I'll just add some further background info as well, the house has been my mums for 19 years and over that time the walls and doors have been repainted twice years apart. She also was a clean freak and would regularly clean around doors and get any dirty marks off the walls so if they'd been there then, they would've been well cleaned off! I've been in the house with my partner just the two of us for 2 and a half years now and because they stand out so much in morning sun, I highly doubt they were there very long before we saw them. Everyone (about 5 people) who's been at ours in the last month we've asked and they all seem confused and some a little afraid of this, so we don't think anyone lying and besides, it's a really random thing to do as some kind of prank.
Has anyone ever had/heard of this happening before and how did the crosses get there? Is it something to be worried about? should we clean them off? And I guess most importantly: What does this mean?