1) As quiet as a mouse 2) Discreet 3) Inobtrusive 4) Invisible 5) Modest 6) Retiring 7) Shy 8) Unnoticeable 1) Discreet 2) Low-key 3) Modest 4) Not conspicuous 5) Not obtrusive 6) Quiet 7) Retiring 8) Unassertive 9) Unassuming 10) Unnoticeable 
• (a.) Not obtrusive; not presuming; modest.
[adj] - not obtrusive or undesirably noticeable
unobtrusive 1. Not conspicuous, blatant, or assertive. 2. Not undesirably noticeable or blatant; inconspicuous.
adjective not obtrusive or undesirably noticeable; `a quiet, unobtrusive life of self-denial`
not obtrusive; inconspicuous, unassertive, or reticent.
[SAT terms] not undesirably noticeable not undesirably noticeable