View source for Sixth sense

As they grow in experience, characters develop a sixth sense enabling them to detect invisible enemies or objects. This ability is applicable against the thief's ability to "hide in shadows" as well as against magical invisibility. Such characters may even notice that they have been targeted by a scrying spell such as clairvoyance.
Probabilities
It is only possible to develop this almost superhuman sense from 7th level and up. In addition to the character's general experience, their intelligence will also contribute to their ability to detect such invisible dangers.
If both level and intelligence are known, the probability that invisibility detection will be activated can be determined based on the following table.
Level
Intelligence (d%)

0-1
2-4
5-7
8-10
11-12
13-14
15-16
17+
7
—
—
—
—
—
—
—
05
8
—
—
—
—
—
—
05
15
9
—
—
—
—
—
05
15
25
10
—
—
—
—
05
15
25
35
11
—
—
—
05
15
25
35
45
12
—
—
05
15
25
35
45
55
13
—
05
15
25
35
45
55
65
14
05
15
25
35
45
55
65
80
15+
15
25
35
45
55
65
80
95

Monsters
Monsters can develop this ability as well, simply based on their Hit Dice instead of level. The intelligence of monsters is described in comparison to the average human. The following terms are used, with their numerical equivalent in parentheses: Non- (0), Animal (1), Semi- (2-4), Low (5-7), Average (8-10), Very (11-12), High (13-14), Exceptional (15-16), Genius (17-18), or occasionally even Supra-genius (19+) .
Some monsters have particularly acute perception. The Game Master should treat such monsters as more intelligent or as of higher level than they actually are for the purposes of detection of invisibility.