Preflight panel Info. shows my placed 300 ppi PDFs at much less effective resolution

Question

Ï'm brand new to InDesign (graphic designer who primarily works in Illustrator, some Photoshop, until now). I understand resolution and PPI but not necessarily in-depth knowledge of the related necessary workflow needed within InDesign, if that makes sense. I've spent hours and hours the last couple days trying to learn the finer points of InDesign as I work on an ad magazine for my company, but I can't for the life of me figure this particular issue out, and I'm guessing (hoping!) it's a very simple solution.
I placed several 300 PPI PDF images in the magazine document (vector PDF's I created in Illustrator, saved as editable PDF's).
I created a Preflight Panel in InDesign with a custom Profile, named it Print Ready, and set image resolution to 250 to ensure as I finish up this magazine design that things are press/print ready. It now shows 5 images with Image resolutions lower than the 250 minimum - all of the pdfs I placed. They all show very low PPI, 74, 109, etc. The 300 PPI pdfs were already sized to the exact proportions they needed to be in the InDesign file and for print, so there was very minimal scaling to fit the images into their frames, so I'm assuming scaling couldn't have created this issue and it has something to do with my workflow and the way I'm bringing them in.
I'd like to point out that this magazine document was created by another designer, and I'm picking up where they've left off, adding some final images and copy. The other picture images she had in the file appear fine, no errors in Preflight Panel...even some preview stock photo's with the X's across them that appear to be very low resolution (we haven't purchased them and replaced them yet in this InDesign file), so not sure I understand why those are not showing up in my Image Resolution as errors??
Anyhow, at some point I came across the Document Presets, which is currently set to Default - and when I click Define, I noticed it shows the PPI at 72. Does this mean that files I place are "brought in" at 72 PPI, regardless if their original PPI is higher? And if so, how do I change that?
If not, how do resolve or correct? 
Sorry for the very long explanation - just want to give as many details as I can, since I'm not exactly sure which details are important to figuring out the problem. Thanks in advance!!

Answer 1

I've checked the resolution in Photoshop just as a double-check and it shows resolution as 300 PPI. When you say "because they can be mixed" do you mean because elements and graphics with the PDF can be of varying resolutions? I hadn't thought of that, if that's what you meant. I do have some items within my pdf that might be lower resolution.
If these show up in my Preflight Panel as an issue, does this mean they would be flagged also with the Printer? In trying to find a resolution I read several forum discussions about how Printers often run your document through a Preflight process and will send it back when there are low resolution images in the document.
Is there some other way to be sure these images are in the InDesign document as 300 PPI so I can know they will in fact print correctly?
PS - Peter - thanks for the super quick response! And I saw your the highlighted "Exceptional Contributor" on the right sidebar for the InDesign forum - clearly well-deserved! Congrats