The + symbol....

...detecting in all metal today, and many times a + (plus) symbol appeared on the display.
What's that all about?
Does it mean the target was too deep to ID?

Good question

Large steel (rusty or otherwise) ..... like a tin/steel can lid...will cause this. Be careful though; as a co-locate non-ferrous metal and a iron-masking culprit can also cause this.

Many thanks Tom, so would you suggest that I investigate these further by removing some top soil?

Sample several..... until you are confident.